FIG. 5 is a block diagram showing an exemplary power supply unit 100 of the related art. The power supply unit 101 is mounted in a host device 100 implemented according to the Universal Serial Bus (USB) standard, and supplies power to a client device 300 by using a cable 200 interposed therebetween. In addition to the power supply unit 101, the host device 100 includes a protective switch 103 configured to connect/disconnect a power supply line from the host device 100 to the client device 300, and a microcomputer 102 configured to control an ON/OFF operation of the protective switch 103.
This type of power supply unit is disclosed in, for example, Japanese Patent Laid-Open Publication No. 1-110-225105.
In the power supply unit 101 according to the related art, in order to maintain an output voltage Vo1 (which is an output voltage obtained from an upper stream side, i.e., power supply unit 101 side of the protective switch 103) output from the power supply unit 101, at a predetermined target value, a voltage feedback control operation is performed. However, in the voltage feedback control operation, neither an output voltage Vo1′ (which is an output voltage obtained from a lower stream side, i.e., cable 200 side of the protective switch 103) output from the host device 100 nor an output voltage Vo2 eventually supplied to the client device 300 is considered.
Thus, in the power supply unit 101 according to the related art, when an output current Io supplied to the client device 300 from the host device 100 is increased, a voltage drop (Io×Ron) generated from an ON resistor Ron of the protective switch 103 and a voltage drop (Io×Rim) generated from a wiring resistor Rim of the cable 200 are increased. In addition, the output voltage Vo2(Vo1−Io×(Ron+Rim)) finally supplied to the client device 300 falls below a lower limit value of a USB standard range (see FIG. 6).